Purification of chromium plating solutions using electrodialysis is well-known in the art (see U.S. Pat. Nos. 3,481,851; 3,909,381; and 4,006,067, the disclosures of which are hereby incorporated by reference). Electrodialysis is the transport of ions through an ion permeable membrane as a result of an electrical driving force, and the process is commonly carried out in an electrodialysis cell having an anolyte compartment and a catholyte compartment separated by a permselective membrane. The permselective membranes are not unlike ion exchange resins in sheet or membrane form. They comprise a matrix of a chemically inert resin throughout the polymer lattice of which are distributed chemically bound anionic or cationic moieties having fixed negative and positive charges. Anion permeable membranes have positive (cationic) fixed charges distributed throughout the polymer lattice and, as the name implies, are permeable to negatively charged ions and are relatively impermeable to positively charged ions. Unfortunately, there are no known anion permeable membranes that are 100% impermeable to cations, and there are no known cation permeable membranes that are 100% impermeable to anions. As a result, there is always in every electrodialysis process some small degree of reverse migration of cations through the anion permeable membrane and/or of anions through the cation permeable membrane.
U.S. Pat. No. 3,481,851 teaches that the dissolved metallic contaminants can be removed from the aqueous chromium plating solution by electrodialysis. An electric current is passed between the anode and the cathode of the cell through the aqueous solutions contained in the anolyte and catholyte compartments of the cell. The electric current causes the contaminant metal cations (for example, iron and copper ions) present in the chromic acid solution to migrate from the anolyte compartment through the cation permeable membrane into the catholyte compartment, reverse migration of anions (for example, chloride ions) being prevented, in theory at least, by the cation permeable membrane. This process effectively reduces the concentration of contaminant metal cations in the chromic acid solution to acceptable levels. In addition, the electrolytic oxidizing conditions prevailing in the anolyte quickly oxidizes the trivalent chromium present therein to the hexavalent state, thereby reducing the ratio of trivalent to hexavalent chromium to an acceptable level. However, the cation permeable membrane also permits the reverse migration of a small amount of mineral acid anions (e.g., chloride or sulfate anions) from the catholyte to the anolyte compartment and as a consequence there is a fairly rapid build-up of these anions in the chromium plating solution. The build-up of mineral acid anions in the anolyte quickly renders the chromic acid solution unsuitable for chromium plating. Therefore, while this process will effectively remove harmful metal cations (for example, iron and copper ions) from the chromium plating solution, it also results in the rapid build-up of equally harmful mineral acid anions (for example, chloride ions) in the plating solution. As a result, this process does not provide a satisfactory solution to the problem of rejuvenating chromium plating solutions by the removal of contaminant metal cations therefrom.
U.S. Pat. No. 3,909,381 teaches that the metallic contaminants can be removed from the chromium plating systems in an electrodialysis cell wherein the catholyte comprises an aqueous solution of at least one ionizable organic compound and wherein the anions of the ionizable organic compounds in the catholyte are oxidized to gaseous oxidation products and water when reacted with the chromic acid-containing anolyte thereby reducing the anion contaminants in the anolyte. However, the oxidation of the organic compound results in the reduction of hexavalent chromium to a lower valent chromium which has an adverse effect on the plating performance of the chromium solution. In addition the electrical conductivity of aqueous solutions of organic compound salts is low and, in turn, limits the capacity and electrical efficiency of the electrodialysis cell.
In electroplating of metals for example, using chromic acid, sulfuric acid is added to the plating solution. A typical plating bath would contain about 250 gram/liter of chromic acid and 2.5 grams/liter of sulfuric acid. The concentration of sulfuric acid relative to chromic acid concentrations increases as the chromic acid is electro deposited. The sulfuric acid concentration is generally controlled by the addition of barium carbonate to the plating solution for precipitation of the sulfate ion or by controlling the drag-out of plating solution into the rinse water. If the rinse water is evaporated, concentrated by reverse osmosis or conventional electrodialysis using both cation and anion membranes or if the rinse water is treated with ion exchange resins, there is no significant separation of sulfuric acid from the chromic acid. This, in turn, precludes operation of a closed-loop chromic acid plating system with current technology unless the chromic acid added as make up to the plating solution does not contain sulfate ions.
Chromium trioxide (chromic acid anhydride, chromic acid) is produced by the reaction of sodium dichromate with sulfuric acid or by adding a large excess of sulfuric acid to a concentrated solution or slurry of sodium dichromate. These processes produce chromic acid contaminated with sulfate ion.
As indicated above, the cation membrane permits the reverse migration of a small amount of anions. The reverse migration of anions through cation permeable membranes increases with increasing concentration of anions in the catholyte solution. The reverse migration of anions does not significantly affect cell performance in electrodialysis wherein the anion in the anolyte and catholyte is the same. However, reverse migration of an anion, for example, a hydroxyl ion, through a cation permeable membrane wherein a multivalent metal cation is simultaneously migrating through the cation membrane from the anolyte to the catholyte can result in precipitation of metal hydroxide in the membrane which can, if in sufficient quantity, cause mechanical damage to the membrane and loss in ion transport capacity. It is well known that reverse migration of hydroxyl ion in the electrolysis of sodium chloride which contains low concentrations of calcium ion causes precipitation of calcium hydroxide in the cation membrane and loss in ion transport.
In electroplating, mining and finishing of metals the aqueous solutions contain salts of multivalent metal ions such as cadmium, chromium, zinc and nickel which are classified as toxic materials. To meet pollution standards, these metals must be removed from the wastewater. It is common to treat the waste solutions with lime and other chemicals to form a sludge which is separated from waste in the solution and disposed in sludge ponds or on land fill. The waste solutions are, at times, further treated with ion exchange resins to remove traces of the toxic metal ions.
The high cost of replacing the electroplating chemicals lost in the waste treatment processes and the high and increasing cost of waste treatment and disposal of the waste dictate the need for a process which permits the recovery for reuse of the electroplating chemicals preferably a process offering reductions in energy waste treatment cost and in the quantity of waste for disposal.
Current processes directed to reducing the loss of electroplating chemicals in rinse water all operate on the same basic prinicipal of concentration of the dilute solutions to the degree that the solution can be returned to the plating bath. None of the processes provide for removal of metal cations and anionic impurities in a closed loop, continuous systems and, therefore, ion exchange or other techniques are required to prevent build up of impurities to levels affecting quality of the finished metals. The current commercial processes include evaporation, reverse osmosis, ion exchange and electrodialysis. Evaporation is broadly applicable but with high energy cost and high investment for corrision resistant equipment. Reverse osmosis is severly limited in use by rapid deterioration in performance of the separating membrane. Ion exchange is suited for processing dilute solutions but a major drawback is that the resin must be regenerated after its ion exchange capacity has been exhausted. Regeneration complicates operation, adds to the waste load, and requires a solution concentration step for return of the chemicals to the plating bath.